policy/v2: write the SSH check period as time arithmetic

Express the maximum SSH check period as 7 * 24 * time.Hour instead of a
bare nanosecond constant, matching how the rest of the code spells out
magic durations.

hscontrol/policy/v2/types.go

@@ -63,7 +63,7 @@ var (
 // SaaS imposes no minimum (0s is accepted) so headscale matches.
 const (
 	SSHCheckPeriodDefault = 12 * time.Hour
-	SSHCheckPeriodMax     = 168 * time.Hour
+	SSHCheckPeriodMax     = 7 * 24 * time.Hour
 )
 
 // ACL validation errors.